In a breathing apparatus, be it for surface or underwater use, what is usually referred to as the "regulator" or "demand valve" or sometimes as the "pressure reducer" is a group of automatic valves which acts as an intermediate stage between a cylinder or cylinders which supply compressed air and a mouthpiece of the breathing apparatus itself, which is generally referred to as a demand type breathing apparatus. Hereinafter the expression "pressure reducer" will be used for this valve group. Broadly speaking, pressure reducers can be divided into two categories, namely single-stage and two-stage reducers, according to whether the said group consists of one valve or two. In the latter case, the first stage consists of an automatic valve which is mounted directly on the outlet from the cylinder and which is adapted to bring about a reduction to what is termed a "medium pressure", which is generally around 8 bars above ambient pressure, while the second stage, which is usually associated with the mouth piece, consists of a valve to which the medium pressure is applied and which reacts in a variable and complex fashion to respiratory suction from the user in order to ensure that the mechanism of breathing is subject to the minimum fatigue.
However, these known two-stage pressure reducers have disadvantages both at the medium pressure stage and at the stage associated with the mouthpiece. In the second stage, it is a familiar fact that the hermetic closure of the valve member on its seating is brought about by the slight predominance which an opposing spring has over the pressure exerted on the valve member by the air at medium pressure. During manufacture and setting, and to avoid the leaks which might eventually occur, the medium pressure is held to lower levels than is necessary, the consequence of which is that the force required to lift the valve member is substantially increased. In practice, the medium pressure never remains constant over time, this being the result of changes which, although minimal, affect the operation of the first stage and which are due to friction, distortion of the springs which decide the valve setting, distortion of the sealing joint which forms the seating in the valve nozzle, and other causes.
Given that, in time, these changes could result in air leaks, the medium pressure, which is set only once and almost always in the course of manufacture, is adjusted to allow the spring a marked predominance, which in the final analysis means that an excessive force is required, to lift the valve member, this being of the order of several grammes per square centimeter in many commercially available pressure reducers.
What is more, this disadvantage, namely the fixed setting, meaning by this a setting which can only be made in the workshop or when the pressure reducer is being manufactured or repaired, prevents the level of the medium pressure from being increased, as may be necessary in certain specific working conditions or at great depths to procure a higher throughput of air and in order better to overcome the increase in friction which is caused when the air inhaled is of greater density. This obviously implies that the setting should be capable of being changed in use by a user who is going deep.
As regards the second stage of known pressure reducers, it is necessary to call to mind the characteristic structure of these members, in which a diaphragm box carrying the tube for connection to the mouthpiece holds the cartridge for the medium-pressure output valve, which has a tube for connection to the flexible medium pressure connector which projects out of the box. The air passing through this valve is admitted into the diaphragm box in various ways and in the box there also operates a sensitive lever member which reacts to movements of the diaphragm. The latter flexes at the operative threshold of inhalatory suction and acts correspondingly when the spring-loaded valve mounted inside the cartridge opens. In general, the air, whose flow is throttled as it passes through the said valve mounted in the cartridge, is conveyed into the box through a vent hole situated in the cartridge itself and may even be guided towards the mouth of the tube connecting up with the mouthpiece.
Depending on the position of this opening and of the means for guiding the air which may possibly be associated with it, the pattern of the dynamic fluid pressures within the box and against the diaphragm may be considerably altered, to the point of having substantial, and even opposing, effects on the behaviour of the diaphragm and thus on the reaction of the second stage to the inhalatory suction.
To be more exact, as is once again well known, if the said opening is allowed to open into the box haphazardly (and if the common case where it is even turned towards the diaphragm is ignored) the depression caused by inhalation may be followed by an overpressure in the box, as a result of the introduction of an excess of air which flows through the said opening from the medium pressure valve. The result is that in the course of one and the same inhalation phase the said valve, instead of opening smoothly or regularly and continuously, opens and closes several times in an intermittent fashion at a rate which may be as much as several cycles per second, which in substance means that the diaphragm operates under vibratory conditions.
Of course, as is generally the case in many other servo-control devices, the vibrations depend on a large number of operating parameters.
In comparison with ideal conditions when the reducing valve in the second stage opens continuously, this intermittent throughput results in a sort of "braking" during inhalation which proves most unpleasant for the user.
In addition, operating conditions may occur which are diametrically opposed to this "braking" when the outgoing flow from the said opening in the cartridge of the pressure reducer is guided axially by the opening itself towards the mouth of the inhalation orifice of the mouthpiece. This being the case, as soon as there is the least sign of inhalatory suction (of a level higher than the operative threshold of suction), the effect of the flow of air emerging from the said opening directly supplements the suction itself, as a result of the Venturi effect, and thus boosts its effect on the flexing of the diaphragm to the limiting conditions where the said Venturi effect persists even beyond the end of the inhalatory movement which initiated it. Under these conditions, the outflow produced has a tendency to continue indefinitely and to bring it to an end would require an opposing pressure which may be produced by for example moving the tongue towards the orifice of the mouthpiece or by closing the glottis, given that under these conditions where outflow is self-maintaining it is no longer necessary to generate inhalatory forces since the pressure reducer itself injects air into the lungs at a more or less high pressure.
In the case instanced in the previous paragraph, the inhalatory force required is reduced to zero but control of the outflow becomes a rather complicated matter. However, the disadvantage of the self-maintaining nature of the flow after it has been started has been corrected in a fairly simple way by providing one or more additional air outlets which are orientated in the opposite direction from the elbow or pipe which directs the main jet into the orifice of the mouthpiece.
However, the simplicity of this solution, which has been adopted in an attempt to remedy the above disadvantage, is not matched by corresponding simplicity and reliability in operation. In particular, the expedient of reducing the injection or Venturi effect by providing compensating ports or other openings in the opposite direction from the Venturi, is very much dependent on dynamic flow conditions in the diaphragm box and is found to be very sensitive even to tiny changes in a large number of parameters, such as distortion, defects in the sealing joint for the valve in the second stage, variations in the medium pressure, and others.
To remedy the drawback represented by the dependence of the injection effect on dynamic flow conditions, many manufacturers have thought it preferable to reduce the injection effect virtually to nothing by means of very large compensating openings. This however results in an outflow which is far from meeting physiological respiratory requirements.